1. Field of the Invention
The present invention relates to a light emitting element using an electroluminescent material. Specifically, the invention relates to an element structure of a light emitting element in which a pair of electrodes and an organic compound layer therebetween are stacked.
2. Description of the Related Art
A light emitting element in which electroluminescence (hereinafter also referred to as EL) is produced by using a pair of electrodes sandwiching mainly a layer containing an organic compound has been attracting attention. The light emitting element uses a phenomenon below; a hole injected from one electrode and an electron injected from the other electrode recombine to excite luminescence center, thereafter light is emitted when luminescence center back to a ground state. That is, the light emitting element is formed so that organic compounds with different carrier transport characteristics are stacked between a pair of electrodes, and holes are injected from one of the electrodes and electrons are injected from the other electrode. The degree of a work function of a material for forming an electrode is regarded as an indicator of hole and electron injection characteristics into an organic compound. A material having high work function is preferable for an electrode where holes are injected, and a material having low work function is preferable for an electrode where electrons are injected.
Conventionally, indium tin oxide (ITO) having a work function of about 5 eV is used for an electrode where holes are injected, which is called an anode, and the anode has been made to contact an organic compound with high hole transport characteristics. Meanwhile, a material in which alkali metal or alkaline earth metal such as Li or Mg is contained in aluminum or the like is used for an electrode where electrons are injected, which is called a cathode, and the cathode has been made to contact a material with high electron transport characteristics.
The organic compound is used for other than a hole injection layer formed of copper phthalocyanine (CuPc), a hole transport layer formed of 4,4′-bis-[N-(naphthyl)-N-phenyl-amino]biphenyl (α-NPD) which is an aromatic amine material, and an electron injection layer or an electron transport layer of a tris-8-quinolinolate aluminum complex (Alq3); a light emitting layer in which a guest material such as quinacridone is added to a light emitting material such as Alq3 and rubrene or a host material.
It is required to improve the characteristics of a light emitting element that electrons and holes are efficiently injected from electrodes, the injected charges are efficiently transported to a light emitting layer, efficiency of recombination of electrons and holes is increased, and the emission efficiency after the recombination is improved.
However, in the case of a light emitting device having a conventional structure in which an organic compound is stacked over an electrode formed of an inorganic compound, sufficient luminance can not be obtained. Further, such a light emitting element has problems to be solved with respect to stability such as high power consumption and short half life of luminance.